The Ultraviolet Light Curve of LMC X‐4: X‐Ray Heating of the O Star and Accretion Disk

We present an archival study of ultraviolet spectra of the X‐ray binary LMC X‐4 obtained with theIUEsatellite. We fit these spectra with models that incorporate the X‐ray heating of the O star and accretion disk. A systematic analysis of the UV continuum confirms the general picture of an O star that fills its Roche lobe and is heated by X‐rays from the neutron star. About 5% of the UV emission arises in an accretion disk, which may precess with a ≈30 day period. We measure the mass transfer through the accretion disk and attempt to relate this quantity to changes in the pulse period and disk precession. The principal source of uncertainty in our model results from the choice of the 30 day disk precession ephemeris. We examine theRXTEAll‐Sky Monitor data for LMC X‐4 to attempt to clarify the variability of the 30 day period. We find no evidence for additional heating of the O star or disk as the result of an X‐ray flare observed withIUEandEXOSATin 1983.